Issue with Pre-Order plugin

First of all, I want to tell you that this plugin is amazing compared with other I have tested.
Your plugin is amazing.

I have an issue though and i don’t understand why is not working.
Everything works fine except one thing.

Let’s say I have 2 pair of shoes sizes 40 and 41. Size 40 is set as a pre-order product and size 41 is not. When I click on size 40 the the button text becomes “Pre-Order” which is correct.
If I click on size 41, which is not set as a pre-order product the button text stays as “Pre-Order” instead of becoming “Add to cart”. The text button does not switch from “Pre-Order” to “Add to cart”.
What am I doing wrong?
I would really appreciate your help